Several publications and patent documents are referenced in this application in order to more fully describe the state of the art to which this disclosure pertains. The disclosure of each of these publications and documents is expressly incorporated by reference herein.
Melanin is a dark pigment found in plants and animals that protects against ultraviolet radiation and provides decoration in the skin, eyes, hair, and fur of animals (reviewed in Riley, P. A., 1997, Int. J. Biochem. Cell Biol 11:1235-39). There are two different types of melanin: brown/black eumelanin and yellow/red pheomelanin. Melanocytes are cells of the epidermis specialized to produce melanin. A sophisticated intercellular signaling system determines whether an individual melanocyte will produce melanin or pheomelanin (reviewed in Brilliant, M. H. and Barsh, G. S., 1998, in The System: 217-29, Oxford University, J. J. et al., eds)).
Melanocytes synthesize melanin inside of specialized organelles called melanosomes (reviewed in Orlow, S. J., 1998, in The Pigmentary System: Physiology and 97-106, Oxford University, New York (Nordlund, J. J. eds.)). Melanosomes are formed by the fusion of two types of vesicles. One type of vesicle, called a premelanosome, apparently derives directly from either the smooth endoplasmic reticulum or the trans-Golgi network. The other type of vesicle derives from the network. Each of these types of vesicles contributes proteins to the melanosome necessary for its function.
Defects in the production of melanin result in pigmentation deficiencies such as albinism. Genetic analysis of abnormally pigmented strains of mice has identified more than 60 genes necessary for the normal production of melanin (reviewed in Silvers, W. K., 1979, The Coat Colors of Mice: A Model for Mammalian Gene Action and Interaction, Springer-Verlag, Basel). One of these genes encodes the enzyme tyrosinase. Tyrosinase protein is a multi-functional enzyme that catalyzes several steps in the production of melanin; tyrosinase activities include the rate-limiting steps of converting tyrosine to (DOPA), and DOPA to dopaquinone (reviewed in Lerner, A. B., and Fitzpatrick, T. B., 1950, Physiol. Rev. 30: 91-126), as well as the oxidation of 5,6-dihydroxyindole to 5,6-indolequinone (Korner and Pawelek, 1982, Science 217: 1163-1165). Both humans and mice lacking tyrosinase activity suffer a severe form of albinism.
Two tyrosinase-related proteins (TRP-1, encoded by the mouse brown gene, and TRP-2, encoded by the mouse slaty gene) also are important for melanogenesis (reviewed in Hearing, V. J., 1993, Am. J. Hum. Genet. 52: 1-7). Each of the TRP proteins shares about 40% sequence identity with tyrosinase and with each other. Each of these three enzymes (tyrosinase, TRP-1 and TRP-2) is predicted to contain one transmembrane domain. Together, they form a high molecular weight complex associated with the melanosomal membrane (Orlow, S. J., 1994, 103: 196-201).
Another protein that is important for the production of melanin is the P protein. In mice, it is the product of the pink-eye dilution (p) gene. In humans, it is the product of the P gene. p-null mice produce significantly less melanin than wild-type mice (Silvers, supra). A wild-type human P gene, but not a mutant human P gene, can complement the hypopigmented phenotype of p-null mouse melanocytes (Sviderskaya, E. V., et 1997, J. Invest Dermatol. 108: 30-34). P protein is apparently needed for the production of eumelanin, but not of pheomelanin (Lamoreux, M. L., 1995, Pigment. Cell. Res. 8: 263-70).
Tyrosinase positive oculocutaneous albinism (Ty-pos OCA) or type 2 oculocutaneous albinism (OCA2) is the most common form of albinism worldwide. It results from mutations at the pink-eyed dilution gene (P) (King, R A. (1995) Scriver, C R. (eds) The Metabolic Basis of Inherited Disease, McGraw-Hill, New York. pp 4353-4392; Ramsay, M. et al (1992) Am. J. Hum. Genet. 51:879-84; Rinchik, E. M. et al. (1993) Nature 361: 72-76. Affected individuals have hypopigmented skin, hair and eyes (Manga P. et al. (1999) J. Dermatol. 26: 738-47), and are thus at increased risk of developing UV-induced carcinomas JG. (1989) Clin. Genet. 36 43-52).
Other conditions that can and may be prevented or treated by increased melanin levels in a subject, include conditions where the skin is more sensitive to the deleterious effects of light, such as porphyrias, polymorphous light eruptions, and the like.
Various studies are directed to methods and compositions for increasing melanogenesis. U.S. Pat. No. 5,352,440, for example, is directed to increasing melanin synthesis in melanocytes and increasing pigmentation by administration of certain diacylglycerol compounds. U.S. Pat. No. 5,532,001 is directed to increasing pigmentation in mammalian skin via administration of certain DNA fragments. U.S. Pat. No. 5,554,359 is directed to increasing levels of melanin in melanocytes by administration of lysosomotropic agents. U.S. Pat. Nos. 6,750,229 and 6,995,804 are directed to the identification of protease-activated receptor-2 (PAR-2) pathway and nitric oxide synthesis modulators, respectively, and their use in modulating pigmentation levels.
Additionally, a wide variety of natural products are known to alter the melanin synthesis in melanocytes. A number of alkaloids, in particular indole alkaloids, are known to decrease the melanin synthesis in melanocytes. For example, U.S. Pat. Nos. 6,110,448, 6,096,295, 5,989,576, 5,919,436 and 5,879,665, all by the patentee in U.S. Pat. No. 5,554,359, describe yohimbine, an indole alkaloid, as skin-lightening agent. However, so far none of these indole alkaloids have been reported to be useful in increasing melanin synthesis.
Despite the development of the noted compositions and methods for promoting skin darkening and melanogenesis, there remains a need in the art for the development of less toxic, safer alternatives to the agents and techniques that are presently available. It is accordingly toward the satisfaction of that need that the present invention is directed.